I'm planning to upgrade my Galaxy S2's OS from android 2.3 to 4.0. This is an official upgrade from Samsung.

It sounds like it's an easy process. Just download the Kies program from Samsung and follow the instructions. But as I've never done it before on an android, would I still have all my apps and data? Or will I have to redownload them all again?

You don't usually have to wipe your data from a carrier upgrade. However, it's possible to have issues requiring data to be cleared for specific apps, and it's also possible that the upgrade will wipe your data for you.

Never fear, though. If you have the current version of Play Store on your phone now, then when you update to 4.0, even if you have to wipe your phone, all of your apps will be re-downloaded automatically (just don't interrupt the process when it starts). Data for apps which use Google Backup Service will also be restored, though only a small fraction of apps use this service.

I've read some say they've kept their data, but I've read others that say they got wiped. The best way to handle it is really to root and take a ROM backup first, but short of that, just back up as much as you can beforehand. There are apps to back up your text messages without root. The only way that I'm aware of to back up specific app data also requires root. I'll note here that anything on your SD card and internal memory should be safe, but it's always a good idea to back that up too.

Given that it's a fairly major upgrade, I wouldn't be surprised if it wiped your data deliberately. ICS is significantly different from Gingerbread, which means there could be a bunch of conflicts and incompatibilities. When flashing custom ROMs, it's generally recommended/required to wipe data first. That doesn't mean it will happen with this update, but it makes me wonder how reliable things would be if they didn't wipe.

The only time I lost any data when upgrading is when I manually wiped all the data.
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Ryan ConradJul 15 '12 at 4:31

2

One of the advantages of official updates is that they deliberately change the db schemas to the same as the old one.
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Richard BorcsikJul 15 '12 at 8:39

@Richard Borcsick thanks for the clarification. I hadn't heard that before. Is that pretty standard across vendors?
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ND GeekJul 15 '12 at 11:44

I've seen this done by HTC and Samsung, so I guess it is. That's how they ensure that you don't need to wipe between updates. (And this is why you might need to wipe when you use custom roms - they use different schemas, so when the app tries to read it, it crashes. )
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Richard BorcsikJul 15 '12 at 11:52

If the update works as it should (which is very likely), then you will still have all your apps and data. However, since some apps may have been upgraded in 4.0, their setting options may change. So after the upgrade, it's a good idea to check your settings.

A few apps (usually games which no longer get updates) are not compatible with android 4.0 so you might lose 1 or 2.

However, it's always safest to backup in case something goes wrong. Seeing as you will be using Kies for the upgrade anyway, you might as well use Kies to back up as much of your data as possible. You can backup contacts, SMS, calendar, music, photos, settings and more.

I'm not sure if Kies backs up your Downloads folder. If it doesn't you will need to manually copy that folder to your desktop.

If the upgrade deletes your apps they will be reinstalled by Google Play as soon as you login. Your apps will be backed up on Google Play, but the settings and data will not (usually). So you would lose your game data, for example.